Self-navigated three-dimensional cardiac T2 mapping at 3T

Background Cardiac T2 mapping using a variable T2 preparation module (T2Prep) has recently gained attention for its ability to quantify the extent of edema (Giri, JCMR 2009). Due to time constraints, the T2 maps are commonly acquired as one or several two-dimensional slices, while the underlying pathology has a three-dimensional (3D) structure. The next logical step would therefore be to exploit recent hardware and software advances to directly acquire 3D T2 maps. To this end, we tested the feasibility of using a self-navigated 3D radial acquisition with a variable T2Prep for 3D T2 mapping at 3T.

Ruud B van Heeswijk 1,2* , Davide Piccini 2,3 , Hélène Feliciano 1,2 , Juerg Schwitter 4 , Matthias Stuber 1,2 From 16th Annual SCMR Scientific Sessions San Francisco, CA, USA. 31 January -3 February 2013 Background Cardiac T 2 mapping using a variable T 2 preparation module (T 2 Prep) has recently gained attention for its ability to quantify the extent of edema (Giri, JCMR 2009). Due to time constraints, the T 2 maps are commonly acquired as one or several two-dimensional slices, while the underlying pathology has a three-dimensional (3D) structure. The next logical step would therefore be to exploit recent hardware and software advances to directly acquire 3D T 2 maps. To this end, we tested the feasibility of using a self-navigated 3D radial acquisition with a variable T 2 Prep for 3D T 2 mapping at 3T.

Methods
Approval was obtained from the institutional review board. A 3D self-navigated undersampled balanced steady-state free precession (bSSFP) sequence (TR/ TE=2.6/1.33ms, matrix 128 3 , flip angle 70°) with a spiral phyllotaxis radial 3D trajectory (Piccini, MRM 2011) was implemented on a 3T clinical system (Skyra, Siemens AG). This self-navigated pulse sequence allows free breathing acquisitions with 100% scan efficiency, while ECG triggering every 2 heartbeats and TE T2Prep =60/30/ 0ms allow for a total acquisition time of~18min with an isotropic spatial resolution of (1.7mm) 3 . The datasets were registered using 3D affine registration (Studholme, Med Image Anal 1996). Through Bloch equation simulations, the heart-rate-dependent T 1 -relaxation-related offset in the T 2 -fitting equation was ascertained. Subsequently, the validity and accuracy of the T 2 fitting was tested in a phantom whose "true" T 2 values were previously determined. The in vivo robustness of the T 2 determination was then tested in 9 healthy adult subjects. Finally, the sequence was applied for the detection of edema in a 75-year-old male infarct patient after revascularization of his proximal left circumflex.

Results
The Bloch equation simulations of the pulse sequence demonstrated that the input T 2 value could be accurately fitted from the magnetization M with the equation [M=M 0 e -TET2Prep/T2 +0.08M 0 ], while the fitted T 2 had only a~3% variation over the common range of heart rates (Fig.1A). The phantom T 2 maps demonstrated high homogeneity and fitting accuracy with the 3D sequence matching the 'true' value to within 1% (Fig.1B). The volunteer study ( Fig.2A-C) suggested good agreement with previously reported T 2 values at T 2 =39.3±3.9ms (Van Heeswijk, JACC Imaging 2012, in press). A region of significantly elevated T 2 (60.4±9.1 vs. 41.0±4.5ms) was identified in the patient in the infero-lateral myocardium of the left ventricle (Fig.2D,E), consistent with the findings on X-ray coronary angiography.

Conclusions
The proposed technique provides an easy and time-efficient way to obtain accurate isotropic T 2 maps of the whole heart. Accurate T 2 values were obtained in the phantom, while those in volunteers are consistent with previously reported values. The preliminary patient study demonstrated elevated T 2 in the infarcted region as expected.  that the dependence of the T 2 fit of the magnetization on the heart rate due to varying T 1 relaxation is relatively low (between 43 and 46ms, a variation of 3%), while the "true" input T 2 was 45ms. B) T 2 map of a phantom that approximates arterial blood and myocardium. The T 2 values of the two 'myocardium' compartments (turquoise) are very similar at 35.3±2.1ms and 35.5±2.4ms and within 1% of the "'true" T 2 value of 35.6ms.

Figure 2
A-C) Axial, sagittal and coronal multi-planar reformatted T 2 maps through the LV of a healthy volunteer. The myocardium is well defined and T 2 =41.3±2.1ms. D) A sagittal T 2 map of a patient with a subacute myocardial infarction demonstrates elevated T 2 =62.4±9.2ms in the inferior and infero-lateral segments (arrows). E) 3D segmented LV at a sub-endocardial surface as seen from a posterior position, with a clearly visible inferior and infero-lateral infarction.